Hitherto, polyimides have been widely employed as molding materials, composite materials, electrical and electronic parts, etc., by virtue of excellent heat resistance as well as excellent properties such as mechanical characteristics, resistance to chemicals, electric characteristics, etc. In recent years, electrical and electronic devices have been drastically down-sized, thinned, and reduced in weight, and plastic film parts for use in the devices are required to have higher long-term reliability in terms of heat resistance, mechanical properties, and dimensional stability.
One known technique for enhancing mechanical properties and dimensional stability of polyimide film in a plane direction is stretching of unstretched polyimide film. For example, Patent Document 1 discloses a technique for enhancing dimensional stability. In the disclosed technique, a polyamic acid solution produced through reaction between aromatic tetracarboxylic dianhydride and aromatic diamine is applied onto a flat plate to form film; the film is heated so as to gradually evaporate the solvent, thereby forming polyimide precursor film; the precursor film is thermally or chemically imidized to thereby form polyimide film; and the polyimide film is drawn at a temperature not lower than the glass transition temperature.
Patent Document 2 discloses a technique for enhancing mechanical properties. In the disclosed technique, a melt-moldable polyimide mixture predominantly containing a polyimide formed from aromatic tetracarboxylic dianhydride and aromatic diamine is melt-extruded to form film, and the film is drawn at a temperature not lower than the glass transition temperature.
Non-Patent Document 1 discloses two different techniques for producing polyimide film having excellent mechanical properties. One technique includes stretching of a polyamic acid film formed from aromatic tetracarboxylic dianhydride and aromatic diamine and subjecting the stretched film to swelling, stretching, and heating for imidization. The other technique includes stretching of polyimide film at a high temperature falling within a glass transition temperature range.
All of the aforementioned conventional techniques are directed to stretching of polyimide films synthesized from aromatic tetracarboxylic dianhydride and aromatic diamine. These aromatic polyimides (formed of all aromatic units) synthesized from aromatic tetracarboxylic dianhydride and aromatic diamine exhibit excellent heat resistance and mechanical properties. However, these aromatic polyimides are colored (pale yellow to reddish brown) by large absorption of visible light. Since a wider variety of functions are expected in the aforementioned electrical and electronic uses, provision of transparent film exhibiting high heat resistance is envisaged.
Non-Patent Document 2 discloses that when a polyimide is produced from an alicyclic monomer, charge transfer between a tetracarboxylic dianhydride moiety and a diamine moiety; i.e., coloring, is suppressed, whereby a colorless transparent polyimide can be produced. However, there have never been reported studies on stretching of a film predominantly formed from polyimide produced from an alicyclic monomer polyimide and reduction in dimensional changes.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 61-296034    Patent Document 2: Japanese Patent No. 2594396    Non-Patent Document 1: “Techniques and evaluation of stretching of plastic film,” Technical Information Institute Co., Ltd. p. 247-252 (1992)    Non-Patent Document 2: “New Polyimide—Fundamental and Application,” edited by Japan Polyimide Association, NTS Co., Ltd. p. 387-407 (2002)